As others have pointed out, the spacers don't modify the rims. A one-piece wheel's characteristics are set when manufactured.(sorry if this is a noob question) If the wheels are offset 25, how exactly does the spacer help? Would a 12mm spacer bring the gap down to 13?
The spacers modify the mounting surface, pushing it further out from the centerline of the car.
The effect of buying an otherwise identical wheel with an offset that is 10 mm lower (ET 15 vs. 25) is the same as using a 10 mm spacer, but with fewer complications (longer wheel bolts, centering issues, etc.).